It is highly desirable to vary the sections of phase fronts of a light beam. For example, as shown in U.S. Pat. No. 4,281,904, there is a device having a crystal formed of electro-optic material such as lithium niobate and lithium tantalate and a patterned array of electrodes formed on the crystal surface. By changing electrical signals to the electrodes, an electric field is selectively produced inside the crystal which alters the refractive index by the electro-optic effect and which selectively modifies the phase front of the incident light beam within the crystal. The modified phase front of the exiting light beam from the crystal is focused to a light stop or to an aperture, which selectively removes portions of the light beam. Thus, the beam of light can be used to print an extended line. It is well known that optical power densities in excess of a few hundred watts/cm.sup.2 gives rise to optical damage within the crystal and can cause nonlinear effects in such crystals. For applications that require high optical power levels, this device as shown in U.S. Pat. No. 4,281,904, is ineffective. This is particularly so in printing applications which require high optical power densities necessary for thermal dye transfer printing applications.
More particularly, the apparatus shown in U.S. Pat. No. 4,281,904 modulates a plurality of regions along a line of light by varying the voltage on the electrodes formed on a TIR (total internal reflection) device are well known. In particular, an electrical signal pattern, which includes a plurality of separate electrical signals, is converted to a selected light intensity profile determined by the electrical signal pattern to achieve a visible display of the signal pattern. The individually addressed element of the electro-optic device acts as a light modulator or gate for one picture element along the recording line. As noted above, this device will have serious optical damage problems when used with a high power or intensity optical light beam.
Referring to FIG. 1, a typical prior art TIR modulator 10 consists of electro-optic material formed of a LiNbO.sub.3 crystal 12. The crystal 12 has three polished surfaces 14, 16, and 18. The surfaces 14 and 16 are arranged such that a collimated beam of light of single wavelength incident at an angle to the plane of the surface 18 is refracted at the surfaces 14 and 16 to incur TIR at the surface 18. It will be appreciated that other crystal shapes are possible to achieve the TIR.
An electrode pattern is deposited on the surface 18 as an array (shown in FIG. 2) with the electrodes 22 parallel to the incident light beam 24. A voltage V, generated by voltage source 25, is applied to the electrode pattern and includes an electric field to adjacent the surface 18 which alters the refractive index of the crystal. With the pattern shown, the modulator 10 behaves in a similar manner to a phase diffraction grating to alter the light output beam, the interdigitating electrodes 22 introducing a periodic electric field which penetrates the electro-optic material 12.
The output beam is diffracted into a series of orders as shown, separated by approximately 2-6 milliradians, whose intensities vary with electrode voltage. For example, if a typically full modulation voltage V of 70 volts is applied to the electrodes, the output light beam contains minimal zero order energy, the energy being transferred to the other orders. Thus, if these orders are stopped by suitable obstacles (a stop or aperture), the incident or original beam direction can be seen to be intensity modulated by the application of the voltage.
As a typical example, the electrodes are 12 .mu.m wide and 3.5 mm long, and the pitch between individual electrodes is 50 .mu.m. Suitable electro-optic materials include LiNbO.sub.3, LiTaO.sub.3, BSN, ADP, KDP, KTP, and Ba.sub.2 NaNb.sub.5 O.sub.15.
In the TIR configurations which have been made available, all the electrodes are joined into two conducting electrode blocks 26 and 28. A signal drive potential V is then applied to electrode blocks 26 and 28. A periodic light phase front with constant magnitude over the area of electrode pattern results from the drive signal.
FIGS. 3 and 4 are top and side views, respectively, illustrating TIR modulator 29 of another prior art device shown in U.S. Pat. No. 4,281,904. In particular, each electrode is connected to its own individual drive voltage while the other ground 33 is grounded or set to some other common voltage. Specifically, individual interdigitated electrodes 30, 32, 34 . . . 36 and 38 have separate drive voltage signals V.sub.1, V.sub.2, V.sub.3 . . . V.sub.j-1 and V.sub.j, respectively, applied thereto and electrodes 40, 42 . . . 48 are connected together and coupled to a common voltage, ground in the embodiment illustrated. It should be noted that the basic structure of modulator 29 is identical to modulator 10 described hereinabove except for the arrangement of the individually addressed electrodes.
Application of different voltage levels to each of the electrodes 30, 32, 34 . . . 36 and 38 will produce a phase modulation of the beam at the location of each electrode, the magnitude of which is related to the individually applied voltage.
It should be noted that the incident light beam 24 preferably spans the electrodes 30, 32, 34 . . . 36 and 38 to essentially fill the full width of the modulator 29. In a preferred embodiment, the light beam 24 is substantially collimated lengthwise of the modulator 29 and is brought to a wedge shaped focus on an internal surface of the modulator 29 which extends widthwise along the electrodes 30, 32, 34 . . . 36 and 38.
To use the phase front modulations to achieve intensity variations in the output (i.e. to provide the line composer), phase microscopy techniques may be used so that each electrode affects the image intensity distribution within a localized region of the image line. This region of the image line corresponds directly to the location of the electrode within the array of electrodes.
Phase microscopy techniques for such conversion are described, for example, in J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill Book Company, New York, 1968, pps. 141-146, including the central dark ground technique described therein. All phase microscopy techniques convert a phase front modulation into a corresponding intensity modulation by spatial filtering, the various techniques differing in the spatial filtering function used.
In TIR devices described above have inherent problems in handling higher light power densities causing beam distortion. These power densities are dependent upon the wavelength of light. For example, in Bulmer et al, "Linear Interferometric Modulators in Ti:LiNbO.sub.3 ", Journal of Lightwave Technology, Vol. LT-2, No. 4, August 1984, describes that optical damage in channel waveguides was observed at 300 W/cm.sup.2 at wavelength at 0.84 .mu.m. Such optical damages is unacceptable in printing devices and other applications.
Davis et al (Nonlinear Frequency Generation and Conversion, SPIE, Vol. 2700, January, 1996) observed a significant decrease in optical damage by periodic reversal of ferroelectric domains.